Novel high-pressure phases and transport properties of IV-VI compounds

Abstract

The IV-VI group compounds have attracted much research interest due to their promising applications in thermoelectric, electronic and photovoltaic industries. In particular, the reports of the record-high experimental thermoelectric properties of SnSe crystals have led to an avalanche of research in improving the thermoelectric performance and exploiting other properties based on the IV-VI binary compounds. At the same time, \textit{ab initio} calculations using the density functional theory (DFT), play a key role in modern computational materials science. The efficiency and effectiveness of first-principles calculations have sparked broad research into theoretical materials design, optimization and discovery. In this dissertation, efforts that have been made mainly involve the following two aspects:

Firstly, theoretical studies on tin monochalcogenides (SnSe and SnTe), based on DFT methods and Boltzmann transport theory, have been conducted to explore the thermoelectric properties. Band engineering has been used in n-type SnSe and p-type SnTe by doping various impurity elements. It was found that the pnictogen group dopants (Sb and Bi) would induce resonant states as well as a delocalized electron density near the CBM in the n-type SnSe, leading to a great enhancement in the out-of-plane normalized power factor in the low-temperature range. Band convergence effects in Zn- and Cd-doped SnTe and a typical resonant effect in In-doped SnTe were observed, which were beneficial to the electrical transport properties. Pressure engineering was employed in pristine SnSe, and an efficient way to manipulate the thermoelectric efficiency of SnSe through external hydrostatic pressures was proposed via the control of phase transition temperature between the $Pnma$ and $Cmcm$ phases.

Secondly, practical applications of \textit{ab initio} evolutionary algorithms have been systematically carried out in different IV-VI systems. New phases and materials with exotic properties were predicted under pressure. In the Sn-Se system, a novel compound possessing an unexpected stoichiometry of 3:4 was determined and experimentally synthesized above 10 GPa. The predicted low-temperature superconducting state of Sn$_3$Se$_4$ occurred at a much lower pressure than previously believed for Sn-Se compounds. In the Ge-Se system, two novel pressure-induced intermediate phases were predicted to exist between $\alpha$-GeSe and $\beta$-GeSe. The $R3m$ phase, transformed from $\alpha$-GeSe, was found to be stable under a low pressure with a robust ferroelectricity analogous to $\alpha$-GeTe. By further exerting pressures, $R3m$ GeSe was predicted to become a 3D topological crystalline insulator $Fm\bar3m$ phase at approximately 6 GPa. In addition to GeSe, Ge$_3$Se$_4$ above 40 GPa and Ge$_2$Se$_3$ with a complex phase transition path between 5 GPa and 30 GPa were also predicted. In the Ge-Te system, a new pressure-induced phase transition path in GeTe was proposed. The $Pnma$-boat phase and the $P4/nmm$ phase were calculated to be stable in the range of 15$\sim$37 GPa and above 37 GPa, respectively. Different from the previously believed high-pressure B2 structure, the newly predicted $P4/nmm$ GeTe was found to have a lower enthalpy due to the low ionicity and the delocalization of electrons. Moreover, our discovery of a cation-rich compound Pb$_3$Te$_2$ above 20 GPa represented the first non 1:1 stoichiometry compound in the Pb-Te systems.